1. Field of the Invention
The present invention relates to novel barrier films and, more especially, to novel barrier films that are resistant to hot water and superheated steam and that are well adopted for the packaging of food materials and perishable comestibles.
2. Description of the Prior Art
To ensure preservation of food products, it is necessary to assure not only protection against dust, but also against moisture, against some gases such as oxygen and carbon dioxide and against odors.
Polymeric resins are employed for this purpose, which are known as barrier resins, which have certain gas and water vapor permeability properties, namely, they are more or less permeable to gases and to water vapor.
Such films typically include one or more layers of polymers which are impervious to gases and of polymers which are impervious to water vapor.
Thus, when permeability to oxygen must be low, polymers are used having barrier properties in respect of this gas which are very high, such as, for example, ethylenevinyl alcohol (EVOH) copolymers, which are known to this art as being excellent materials in this regard.
If a sufficient protection against water vapor is to be provided, a barrier layer of polyolefins such as polypropylene is typically employed.
Since the requirements of imperviousness to gases and to water vapor vary from one form of packaging to another, and from one food product to another, the films employed may include one or more polymer layers which have different gas and water vapor permeability properties.
To improve the quality of "fresh" food products and to increase their preservation time, the food products must be packaged aseptically.
Many processes exist for this purpose. For example, pasteurization and sterilization are representative.
In these processes, the packaging--or the packaged food product--is subjected to the action of hot water (85.degree. C.-100.degree. C.) or to the action of superheated steam for periods of time ranging from a few seconds to several hours. For example, in the case of sterilization, the temperature attains a value of 121.degree. C. or even 135.degree. C. to 140.degree. C. It is then necessary for the packaging, i.e., a film having one or more barrier layers, to withstand mechanically the combined action of water and of temperature, as well as the pressures, which may be as high as 3 to 4 bars.
It is found that at the time of the sterilization, or under the effect of hot water, certain films, especially those based on EVOH, undergo degradation, e.g., manifested as creasing of the affected layer or even a total delamination of the multilayer film.